There is no trail. It's just the river. Fine. A unique opportunity to hike in a river on a warm day.
But you're hiking with 3,000 to 5,000 of your closest friends. Half of whom don't understand or respect difficult trails--and make no mistake, the Narrows is difficult.
On a medium river day, there are few if any points of dry land. There is no place to stop. No place to sit and re-tie a boot. You're in the river, and that's it. At its lowest the river is a foot or two deep. But the river gets deep in places--up to your chest--and you either go in or you go back.
All of that is fine. I'm no cupcake, like I said I've done Angels and pretty much every other day hike in Zion.
But the trail isn't what you think. This isn't a sandy-bottom river. The river bottom is nothing but rocks and boulders. Imagine a million bowling balls of different sizes, all covered in incredibly slippery river slime, under 3-4 feet of water.
That's it. That's the Narrows.

Hours of unending, ankle twisting, dangerous slime-covered rocks under several feet of water. Every step you take you're subject to serious vacation-ending injury with no place to stop, no way to get help.
It's astounding the park service doesn't have to air lift a hundred people a day out for broken bones. That's not hyperbole. Even with experience and in the rented hard-soled water boots and walking staffs, it is ridiculously treacherous. People talk about the chains on Angels being dangerous and not understanding how the park service allows people to attempt a death-defying climb. But I'll take Angels over the Narrows any day.
And the messed up thing is that the views you get in the Narrows aren't unique or even especially good. The park is filled with tall, sheer cliffs and gorgeous scenery. There's nothing you can see in the first few miles of the Narrows that you can't find better examples of elsewhere in the park.
The Narrows is the emperor's new clothes for people who make an 8-hour stop in Zion on their way somewhere else. Cold, dangerous, crowded, and generally unpleasant.